The present invention relates to a liquid crystal display device comprising a substrate made of a flexible material, preferably a plastic foil substrate, and a liquid crystal cell containing a bistable ferroelectric liquid crystal arranged between covering electrodes.
This type of display device has already been described in German Published Patent Application DE 196 49 761.2. Ferroelectric liquid crystals are known for their rapid and bistable switching properties. However displays made using these crystals are usually impact sensitive and react by loosing written information as soon as a material flow occurs in the interior of the crystal layer. In the known foil displays with ferroelectric liquid crystals this problem is minimized by using special thin foil substrates and by using a liquid crystal with a so-called xe2x80x9cbookshelfxe2x80x9d texture.
It is an object of the present invention to provide a ferroelectric liquid crystal display device with a flexible substrate, which has a higher mechanical stability than currently known displays.
This object and others which will be made more apparent hereinafter are attained in a liquid crystal display device comprising a liquid crystal cell containing a bistable ferroelectric liquid crystal arranged between covering electrodes and a substrate made from flexible material, preferably a plastic foil substrate.
According to the invention the optical thickness of the liquid crystal cell is a fourth of the wavelength of light and spacers are arranged between the covering electrodes. Preferably the spacers are at a distance of from 40 to 60 xcexcm from each other.
The shear force that acts on the crystal layer of this display device depends strongly on its thickness. By using a liquid crystal cell according to the invention having an optical thickness that corresponds to a quarter of the wavelength of light, the impact sensitivity is thus clearly reduced. The spacers also increase the mechanical stability of the display device by themselves, so that the display device resists compression, bending and torsion. A diffuser or a reflector can be arranged on the rear side of the display device, which insures that light passes through the quarter wave cell twice. Because of that the liquid crystal cell according to the invention has a thickness, which amounts to about 0.8 xcexcm, which is about half that for cells of the currently known liquid crystal display devices. The spacers arranged between the covering electrodes of the liquid crystal cells can have a thickness of about 0.8 xcexcm corresponding to that of the cells and a diameter of about 15 xcexcm and preferably have plane surfaces on each end or side. These spacers may be made by photolithography or printing techniques. The spacers can however also be provided with an adhesive layer and can be distributed on the covering electrodes. Orienting layers for the liquid crystal comprising SiOx or Ta2O5 layers can be deposited by evaporation or spraying on the cover layers, obliquely or diagonally to the substrate surface. This type of orienting layer exerts a great anchoring influence on the molecules of the liquid crystal material. An additional increase in the mechanical stability may be obtained by producing a low pressure in the liquid crystal cell. The mechanical properties of a display device according to the invention having about 3 cm2 surface area include typically a compressive load over 120 N/cm2, a bending radius of less than 1.5 cm and a switching potential of under 5 V at temperatures of 0xc2x0 C. to 60xc2x0 C.